1. Field of the Invention
The present invention relates to a microelectromechanical inertial sensor, in particular for free-fall detection applications, to which the following treatment will make explicit reference without this implying any loss in generality.
2. Description of the Related Art
As is known, there is an increasingly widespread use, in portable devices, such as for example laptops, PDAs (Personal Data Assistants), digital audio players, cell phones, digital camcorders, portable computers and the like, of hard-disk (HD) units for storage of data. In fact, hard disks have a high storage capacity, a high access speed, and a low cost.
However, hard disks may suffer damage when used in portable applications. In fact, portable apparatuses are very liable to violent impact, in particular when they fall to the ground during their normal use. The impact of the portable apparatus against the ground has harmful repercussions on the hard disk inside it, producing in the worst case permanent damage and consequent loss of the stored data. Hard disks are particularly susceptible to impact, in so far as a read/write head is normally kept at a small distance from a corresponding data-storage medium. Consequently, due to the impact, the read/write head hits the storage medium and can get damaged together with it.
In order to prevent, or at least limit, onset of said destructive events, it has been proposed to use free-fall detection devices coupled to the hard-disk unit. In particular, free-fall detection devices comprise an inertial sensor made with microfabrication techniques (of a MEMS type), adapted to detect accelerations acting on the portable apparatus, and a free-fall detection circuit, adapted to identify the free-fall event on the basis of the detected accelerations. When free-fall is detected, an appropriate interrupt signal is issued to a control device of the hard disk, which, in turn, issues a command for forced “parking” of the read/write head in a safe area, coinciding, for example, with the position assumed with the apparatus turned off.
In a known manner, inertial sensors of a MEMS type comprise suspended structures of semiconductor material, which are fixed to a substrate at one or more anchorage points and are movable along one or more axes of detection. In particular, the suspended structures form one or more mobile masses (referred to in general as rotors), which undergo displacements with respect to the substrate in the presence of external accelerations. Rotor electrodes are associated to the mobile masses and stator electrodes face the rotor electrodes and are fixed with respect to the substrate; the stator and rotor electrodes form detection capacitors, the capacitance of which varies with the displacement of the mobile mass. A suitable interface electronics is able to detect, on the basis of the variation of this capacitance, the amount of displacement of the mobile mass, and so the acceleration acting on the portable apparatus. In particular, the displacement of the mobile mass occurs also in the presence of a static acceleration (for example, the acceleration of gravity), generating a corresponding capacitive unbalancing, which is detected by the interface electronics. It follows that, even in a rest condition, a non-zero acceleration is detected.
Instead, during free fall, the displacement of the mobile mass with respect to a reference system fixed with respect to the substrate, which is also undergoing free fall, is zero (the rotor electrodes remain centered with respect to the corresponding stator electrodes), and the detected acceleration is zero (or substantially zero if air friction is considered). Accordingly, the free-fall detection circuit identifies a condition in which the overall acceleration acting on the portable apparatus (in practice, the vector sum of the components of acceleration acting along three Cartesian axes x, y, z fixed with respect to the portable apparatus, detected individually by a triaxial accelerometer) is lower than a certain threshold, close to zero:a=√{square root over (ax2+ay2+az2)}≦Thwhere a is the amplitude of the acceleration acting on the portable apparatus, given by the vector sum of the components of acceleration ax, ay and az acting along the axes x, y and z, respectively, and Th is the acceleration threshold, having a value close to zero. The presence of a triaxial accelerometer is required for detecting the free-fall event irrespective of the orientation of the portable apparatus during the free-fall motion.
The operations described (in particular the calculation of the vector sum of the individual components of acceleration) are rather laborious to carry out and render the free-fall detection circuit complex. On the other hand, the tendency is known towards a decrease in size of hard disks, in particular in the field of portable apparatuses, in which size miniaturization has a fundamental importance.
Therefore, the need is felt for a simplification of the free-fall detection electronics, not only to obtain a reduction in the manufacturing costs, but above all to reduce the occupation of area.